1. Field of the Invention
The present invention relates generally to direct current discharge lamps and, more particularly, to improvements in a direct current discharge lamp for use in an optical instrument and to an improved light source using such lamp as attached to a reflector.
2. Description of the Related Art
Discharge lamps such as extra-high pressure mercury lamps and metal halide lamps are widely used in optical instruments such as liquid crystal projectors, OHPs and motion picture projectors and in general lightings. Such discharge lamps are highly advantageous in that their energy efficiency is three to five times higher than that of incandescent lamps such as halogen lamps, which emit light by heating filament, and their life time is five to ten times longer than that of such incandescent lamps.
Recently, demands have arisen from users, particularly from users of optical instruments that discharge lamps be further improved in life time, energy efficiency (specifically, to achieve a higher screen brightness per electric power applied to lamps) and evenness of screen brightness.
Intensive study and development have been made to improve discharge lamps for use in optical instruments so as to satisfy such demands; for example, enabling lamps to use direct current in order to enhance their emission efficiency in optical instruments, shortening the spacing between opposite electrodes to shorten the arc length or increasing the pressure in the lamps thereby improving the luminance of arc, and improving the reflecting efficiency of a reflector based on an improved arc luminance.
FIG. 4 shows a conventional discharge lamp (B). This conventional lamp (B) involves the following problems: (1) anode 12b, which is heated to a higher temperature than cathode 12a in DC lighting, is subjected to severe damage and loss, resulting in a substantial luminous flux attenuation from the initial period of use, hence in an unsatisfactory life time (refer to FIG. 5); (2) seal-cut portion 27 of bulb 21a interfere with the light path to cause a 10 to 20% loss in lighting efficiency (refer to Table 1); (3) the seal-cut portion 27 is reflected on a screen as shadow causing uneven screen brightness (refer to Table 2); and like problems.
To solve the problems (2) and (3) of the above problems, study has made to develop tipless lamps which are fabricated without using any sealing tube so as to avoid formation of any seal-cut portion as indicated at 27. Such tipless lamps are now being realized for a lower wattage.
Such tipless technique, however, involves not a few problems remaining unsolved. The first one is unfeasibility of obtaining lamps of a higher wattage due to process limitations. Specifically, a higher wattage lamp requires the use of a glass tube having a thicker wall and a larger diameter and this makes it difficult to achieve tipless sealing. The second one is incapability of preventing malfunction of a lamp due to impurities produced in the lamp. Specifically, in the manufacturing process of even a lower wattage lamp, certain amounts of impurities are produced from a glass tube used. The amounts of impurities grow larger as the wattage of a lamp grow higher because such a higher wattage lamps employs a larger glass tube. Larger amounts of impurities remaining in the lamp cause malfunction of the lamps. The third one is costly manufacture, which leads to expensive optical instruments such as a projector. Moreover, the problem (1) is left unsolved.
It is, therefore, an object of the present invention to provide a direct current discharge lamp having a prolonged life time.
Another object of the present invention is to provide a direct current discharge lamp enjoying an improved energy efficiency.
Yet another object of the present invention is to provide a direct current discharge lamp providing improved evenness in screen brightness.
Further object of the present invention is to provide a direct current discharge lamp of a higher wattage and to enable the manufacturing cost of a direct current discharge lamp to be reduced.